Replace the Amazon SIM with a fully paid up local provider's 3G data dongle SIM. Do a lot of hacks in the system software to disentangle the 3G from Amazon's servers (even if you are paying for the actual 3G connection yourself, using Amazon's server side resources to support it is *NOT* a good idea) and resign yourself to paying your provider for every byte transferred to/from Amazon.
Edit: Not applicable to DX, see replies belowAt the end of all that, you might as well have simply bought a portable 3g => WiFi access point and set up the DX to connect to it, which would have kept your free access to the Amazon store as long as you remember to turn off the portable access point when you want to use the internal Amazon 3G and also avoids all risk of getting your device blacklisted if you miss anything disentangling the internal 3G from Amazon.

This post was written in full awareness of the forum policy and to the best of my knowledge does not help, condone or encourage any action that could damage Amazon's business model.

It seems I am insufficiently acquainted with the model in question. Thank you for the correction

However SIM replacement is possible for any 3G device that has a physical SIM holder so that part of my response still stands. I believe some eInk Kindle users have gone down this road already, and I am myself looking at what is needed to do this for a K3 3G but as some of the essential steps if taken without replacing the SIM are likely to result in violations of Amazon's Kindle Terms of Use, I cannot discuss it further here.